Sonar systems for transmitting and receiving sonic energy advantageously employ a recording device such as the familiar depth recorder chart which provides a continuous visable record of the depth of a reflecting surface, e.g., the sea bottom. As is well known in sonar depth measurements made from a moving ship, the depth measurements will be affected by the heave of the ship caused by ocean waves or swell. The resulting variation of depth measurement on successive depth measurements obscures the small changes in the flatness of the sea bottom by producing a sinusoidal perturbation of depth measurements of a sea bottom which is actually flat. Alternatively, the sinusoidal perturbation masks variations in the sea bottom which may actually exist. An exact knowledge of the nature of the sea bottom is required in many instances such as where pipe or cable is to be placed on the ocean floor since unsupported regions of the pipe or cable between shallower portions of the ocean bottom may induce stresses which cause the pipe or cable to break.
In the prior art a heave sensor located aboard the ship carrying the sonar has been used to sense the heave of the ship at the time of transmit. A correction to the observed depth measurement of twice the heave (the roundtrip correction) is effective for removing within acceptable limits the heave perturbation in those instances where the ship does not have much vertical movement between the time of transmit and the time at which the reflected signal is received, such as in shallow water. However, where the depth of the water is great and the vertical movement of the ship is significant between the transmit time and receive time, a problem exists in the prior art depth measuring systems in that the received signal shows an apparent substantial change in depth of the sea bottom from pulse to pulse which change obscures the nature of the sea bottom.